1. Field of the Invention
This invention relates to a radiation image information read-out system for obtaining image signals from a stimulable phosphor sheet carrying a radiation transmission image stored therein by exposing the stimulable phosphor sheet to stimulating rays and photoelectrically reading light emitted from the stimulable phosphor sheet in proportion to the stored radiation energy upon exposure to the stimulating rays.
2. Description of the Prior Art
In U.S. Pat. No. 3,859,527, there are disclosed a method and a system in which a radiation transmission image of, for instance, a human body is once stored in a stimulable phosphor sheet and the stimulable phosphor sheet is scanned by stimulating rays and light emitted from the stimulable phosphor sheet in proportion to the stored radiation energy is detected by a photo-detector, thereby obtaining image signals for reproducing the radiation trasmission image.
In the system, a half-silvered mirror is positioned at 45.degree. with respect to the stimulable phosphor sheet, and stimulating rays are projected onto the sheet through the half-silvered mirror from behind the mirror. Light emitted from the sheet upon exposure to the stimulating rays is laterally reflected by the half-silvered mirror to impinge upon an image intensifier or a photomultiplier. Alternatively, stimulating rays are projected onto the backside of the stimulable phosphor sheet through an aperture and light emitted from the front side of the same is laterally reflected by a prism to impinge upon an image intensifire. However, this system is disadvantageous in that since the half-silvered mirror or the prism is substantially spaced from the stimulable phosphor sheet, light emitted from the stimulable phosphor sheet upon exposure to the stimulating rays, which is weak and nondirectional, cannot be efficiently accumulated.
In the system disclosed in U.S. Pat. No. 4,302,671, a photo-detector is disposed in close proximity of the stimulable phosphor sheet and a small reflecting optical device is disposed between the photo-detector and the phosphor sheet so that stimulating rays traveling through the space between the photo-detector and the phosphor sheet are reflected by the reflecting optical device to impinge upon the phosphor sheet. Though this system is advantageous in that the receiving solid angle of the photo-detector can be enlarged whereby S/N ratio can be improved, there is required a complicated and precise optical device in order to cause the stimulating rays to travel through the space between the photo-detector and the phosphor sheet and to impinge upon the small reflecting optical device.
In Japanese Unexmined Patent Publication 58(1983)-121874, there is disclosed an X-ray image converter comprising a stimulable phosphor sheet and a photo-sensor utilizing a photoconductive semiconductor. The photosensor comprises a photoconductive semiconductor sandwiched between a pair of transparent electrodes and is laminated on the stimulable phosphor sheet over the entire area thereof. The transparent electrodes may be divided into a plurality of parallel strips. In this X-ray image converter, the radiation transmission image stored in the stimulable phosphor sheet is read out by scanning the phosphor sheet by stimulating rays through the transparent electrodes, or by providing an array of LEDs over the entire surface of the photo-sensor and successively energizing the LEDs to scan the phosphor sheet. This system may be considered to be advantageous in that the S/N ratio can be improved since the photo-sensor is immediately superposed on the stimulable phosphor sheet and therefore there should be less loss of the light emitted from the phosphor sheet, between the phosphor sheet and the photo-sensor.
However, in fact the X-ray image converter suffers from the following drawbacks.
Since the photo-sensor is superposed on the stimulable phosphor sheet over the entire surface thereof, the photoconductive semiconductor of the photo-sensor, which has low resistance to intense light, is apt to be deteriorated by the noise erasure operation (Pertaining to removal of the remaining radiation energy stored in the stimulable phosphor sheet after completion of read-out of a radiation trasmission image, and generally accomplished by exposure to a large amount of light having wavelengths in the stimulating spectrum. This operation is required since the residual radiation energy constitutes noise in the next read-out of image information.), and the stimulable phosphor sheet becomes heavy and cumbersome. Further, it is very difficult to realize a stimulable phosphor sheet bearing thereon such a photosensor and/or an array of LEDs over the entire surface thereof, and such an arrangement would be very costly. Further since the area of the transparent electrodes is very large even if they are divided into parallel strips, excessive dark current will be inherently generated. Due to the excessive dark current and a large capacitance, the S/N ratio cannot be appreciably improved.